Try this for an example:

http://www.eurorailways.com/timetable/index.htm

Fastest way is to go from Avignon via Montpellier and TGVs to Figueras, Spain, where TGV high-speed line currently ends and transfer to trains to Barcelona and from there to Madrid - there is an overnight train Barcelona to Madrid

Or you can take a TGV train to Paris and then hop the overnight Talgo train right to Madrid, saving cost of a night in a hotel - check out these fab IMO sites for more info on trains in France and Spain - www.budgeteuropetravel.com (byron here is someone you can talk to on the phone and will IME of dealing with him for years buying railpasses he will answer questions even if not buying anything!; www.ricksteves.com and www.seat61.com. Check www.voyages-sncf.com for schedule between France and Spain and pricing with some online discounts if you book way in advance. if taking enough trains in those two countries consider the France-Spain Railpass.

http://www.elipsos.com/en/Default.aspx

You can nab some really cheap fares on the overnight Elipsos (talgo) overnight train from Paris to Madrid - and you can also get some deep discounted tickets on the French railway site - could make a very cheap transfer for such a distance and again you save on a night's hotel cost.

If time if of essence consider looking for cheap flights from either Lyon or Marseille or Nimes or any airport to Madrid - no matter how you go by rail it is going to take a long time - all day or all night.

I flew from Barcelona to Lyon on Easyjet, which was great. I know they also have flights from Lyon to Madrid and Barcelona.